Behavioral economic analysis of normalized demand functions is proving to be a useful way to quantify and compare drugs as reinforcers. Demand and response output functions generate two numbers, Pmax and Omax, both of which incorporate scheduled dose and response requirement variables, and likely reflect important aspects of the reinforcing effects of the drug. Because the reinforcing effects of drugs are probably related to their abuse liability in humans, these metrics may allow us to evaluate this aspect of the relative abuse liability of drugs more completely than we can do with most other procedures. The current proposal has as one of its aims the further use and development of the behavioral economic analysis of drugs as reinforcers in rhesus monkey models of i.v. drug-reinforced responding. In particular, the analyses will evaluate the relative reinforcing effects of a number of drugs of abuse using own-price elasticity of demand, will determine how the reinforcing effects of some drugs are modified when these drugs are combined with a second drug of abuse, and will measure demand functions when the animals can choose between two drug options using cross-price elasticity of demand. These latter two studies will provide information about types of polydrug use that are likely to occur commonly in human drug abusers, yet about which we have little data and few ways to study. The results should provide information about some of the behavioral and pharmacological mechanisms that lead people to take drugs simultaneously, or that direct their choice when more than one drug is available. We anticipate that these studies will help to identify and quantify important aspects of drug-seeking and drug-taking behavior and will put us in a better position to understand how these aspects control this behavior and what they might signify for intervention and treatment.